The two projects, currently badged R7 and R8, will provide a mixture of housing, retail, leisure and commercial office space.
Both schemes are due to start next year.
R7 (above) is a new development of 150,000 sq ft. Its West tower will have 13 storeys of offices above a cinema and retail units while the East tower will hold 11 storeys of offices above retail space at ground level.
R8 is a 200,000 sq ft scheme to construct a mixture of private and affordable housing together with office accommodation and retail space.
The contracts are the latest in a long line of projects that Kier is involved in at King’s Cross.
Kier is currently redeveloping the Grade II listed Midland Goods Shed and East Handyside Canopy for supermarket chain Waitrose with offices above for the Guardian Media Group.
Kier recently completed both the £42m ArtHouse scheme, a housing development providing 143 homes for the area; and 5 Pancras Square, the £65m public services hub for Camden Council, heralded as one of the greenest public buildings in the UK.
Haydn Mursell, Kier chief executive, said: “As the London market grows at pace, we are establishing ourselves as a delivery partner of choice on major projects, regeneration schemes and in high-rise development.
“We look forward to continuing to increase our presence in the market.”